Apparatus for manufacturing steel plates coated with aluminium powder

ABSTRACT

Apparatus for rapidly and continuously manufacturing steel plates coated with aluminum powder wherein the plates are cleaned and coated with a slurry of aluminum powder in water, dried, rolled between special internally water-cooled rolls to achieve substantially theoretical density of the coated aluminum and then heat treated at between 450*C and 700*C for 1-5 seconds in a furnace high frequency induction furnace or a radiation tube type furnace or combination of both.

United States Patent Yamagishi et a1.

APPARATUS FOR MANUFACTURING STEEL PLATES COATED WITH ALUMINIUM POWDERInventors: Hidehisa Yamagishi; Takashi Kamata; Yoshio Ujiki; FumitoshiYokoi; Tadao Takahashi, all of Kawasaki, Japan Assignee: Nippon KokanKabushiki Kaisha,

Tokyo, Japan Filed: July 3, 1973 Appl. No.: 376,111

Related US. Application Data Division of Ser. No. 170,063, Aug. 9, 1971,Pat. No. 3,769,068.

US. Cl. 13/5; 13/22; 13/26 Int. Cl. HOSB 11/00 Field of Search 13/26,27, 34, 22, 2, 5;

HOT AIR BLOWER Oct. 14, 1975 [56] References Cited UNITED STATES PATENTS2,130,756 9/1938 Malam 13/2 2,325,638 8/1943 Strickland... 13/26 X2,441,500 5/1948 Miess 13/2 X 6/1971 Seelandt 13/34 X Primary ExaminerR.N. Envall, Jr. Attorney, Agent, or Firm-Flynn & Frishauf ABSIRACTApparatus for rapidly and continuously manufacturing steel plates coatedwith aluminum powder wherein the plates are cleaned and coated with aslurry of aluminum powder in water, dried, rolled between specialinternally water-cooled rolls to achieve substantiallytheoreticaldensity of the coated aluminum and then heat treated atbetween 450C and 700C for l-5 seconds in a furnace high frequencyinduction furnace or a radiation tube type furnace or combination ofboth.

8 Claims, 9 Drawing Figures O ct. 14,1975 Sheet 0f4 3,912,846

U.S. Patent Oct. 14,1975 Sheet2 of4 3,912,846

FIG. 2

6 8 1012145 Time, seconds FIG. 3

lllllllllll Time, seconds APPARATUS FOR MANUFACTURING STEEL PLATESCOATED WITH ALUIVIINIUM POWDER This is a division of application Ser.No. 170,063, filed Aug. 9, 1971, now U.S. Pat. No. 3,769,068.

The present invention relates to apparatus for manufacturing steelplates coated with aluminum powder.

According to the prior art, a steel plate to be coated with aluminumpowder is heated in a medium having very low thermal conductivity as anoxidizing atmosphere, a non-oxidizing atmosphere or a reducingatmosphere.

For example, U.S. Pat. No. 3,382,085 discloses a method of coating 0.10inches thick and 6 inches wide coil with aluminum by heating it in asemi-muffle furnace at 500C for 1 hour. French Pat. No. 66,915 disclosesa method of coating steel plates with a mixture of zinc and aluminumpowders by heating in a 100 percent hydrogen atmosphere either at 3l6454C for 6 hours or at 3l6-357C for 24-48 hours. Japanese PatentPublication No. 40/4131 discloses that a steel plate may be coated withaluminum, subjected to mechanical working with a reduction of 35-70percent and heated at an arbitrary temperature within the range 400700Cfor an arbitrary period within the range of 1 second to 1 hour so as toprevent forming of an alloy layer or to limit such an alloy layer, ifformed, to be less than 3 ,u. thick by heat treatment. According to thedisclosure of Japanese Patent Publication No. 44/20841, a steel platemay be coated with aluminum, by heating in a non-oxidizing atmosphere orreducing atmosphere at a temperature less than that of melting point ofaluminum for a short period of time to perform a preliminary sintering,then at a pressure with a reduction rate of less than 10 percent tocompact the aluminum layer and heating as well as sintering at 350500C.

According to these prior art methods, it takes a considerable time toheat steel plates up to a predetermined temperature (heat treatmenttemperature) because the steel plates are heated in a gas having verylow thermal conductivity. Continuous operation from rolling toheattreatment, consequently, requires heating to an unnecessarily hightemperature or in a furnace having a long oven length. Such a furnacehas a high construction cost, and raising the atmosphere temperature ina short time will also raise the production cost.

In manufacturing steel plates coated with such metal powders as aluminumpowder, steel plates are subjected to a pretreatment such'as degreasingwith alkali, washing in water, pickling, etc.', then coated with aslurry consisting of metal powders and water, and then dried, rolled andheated. In the prior art, it is difficult to perform rapid andcontinuous processing efficiently as respective stages of drying,rolling and heat treatment all require considerable time.

In any of the above prior art processes so much steam is generated indrying the slurry of metal powder coated on the steel plates that dryingspeed is considerably slowed down particularly when drying is carriedout only by means of an induction heating, thus bringing aboutinefficiency. In order to accelerate drying, generated steam must beexhausted rapidly from the furnace and air must be blown into the samesimultaneously. In a heating furnace used for drying slurry, coatings ofmetal powder and for heat treatment after rolling, the heating speeds,temperatures, etc., must be adjustable dependent on the thickness ofstrips, speeds of treatments, kinds of metals for coating andthicknesses of the coated layer. The conventional high frequencyinduction heating furnace has often been used 'only for heat treatment.A continuous heating furnace for strips,- for drying coated layersmainly consisting of aqueous solution, in which temperature isarbitrarily adjustable, has not yet been proposed.

In coating with metal powders, metal powders mixed with water or solventand binder in a form of a slurry are painted or electrostatically coatedon steel plates. In any of these cases, the coated steel plates aresubjected to a heat treatment for drying and sintering the coated layerprior to rolling, and the steel plates coated with metal powder arecontinuously rolled after the L heat treatment.

In the prior art, when rolling such heated steel plates, the surfaces ofthe rolls are cooled and smoothed by means of water or a rollinglubricant oil. However, when the rolls are cooled from the outside themetal powder coated on the steel plates may possibly flow out from thesame thus preventing formation of the desired coated layer. In theabsence of a cooling operation, the surface temperature of the rollsbecomes too high, so that the metal powder easily adheres to the rollsthus it is impossible to obtain a uniform ancl smooth layer. Moreover,the lubricating properties of the grease in the ball bearings is damagedat high temperatures.

The present invention aims to substantially eliminate the defects of theprior art..An object of the present invention is to provide apparatusfor continuously manufacturing aluminum-coated steel plates havingexcellent adherence, wherein steel plates to which aluminum powder isapplied are rolled to comminute the aluminum powders sufficiently and toachieve a nearly theoretical aluminum density (the same density as thatof pure aluminum sheet), thus forming a continuous aluminum layerthereon. The plates are then subjected to a rapid heat treatment.

Another object of the present invention is to eliminate defects of theprior art processes of drying, rolling and heat treatment and to providea rapid and continuous method of manufacturing aluminum-coated steelplates.

Still another object of the present invention is to eliminate defects ofthe prior art heating furnace used for manufacturing steel plates coatedwith metal powder.

Still another object of the present invention is to eliminate defects ofthe conventional rolls and to provide novel hollow compacting rollswherein cooling water is circulated to cool the same and the ballbearings thereof are provided with rotary joints which facilitates thesupply and drainage of water to and from the bearings while the rollsare rotating.

The present invention can be more fully understood from the followingdetailed description taken in conjunction with the accompanying drawingsin which:

FIG. 1 schematically shows the whole process for manufacturing steelplates coated with aluminum powder;

FIG. 2 is a diagram showing conditions for heat treatment; Y

FIG. 3 is a diagram showing the temperature rises in various heatingfurnaces;

FIG. 4 shows a longitudinal section of a high frequency inductionheating furnace;

FIG. 5 is a cross-sectional view of the furnace shown in FIG. 4 takenalong the line VV;

FIG. 6 shows a longitudinal section of a part of hollow rollers.

FIG. 7 shows a longitudinal section of a heating furnace employing highfrequency induction heating and radiation heating;

FIG. 8 is a cross-sectional view of the furnace shown in FIG. 7 takenalong the line VIIIVIII; and

FIG. 9 is a cross-sectional view of the furnace shown in FIG. 7 takenalong the line IX-IX.

EMBODIMENT 1 In FIG. 1 steel strip (A) is transferred to a pretreatmenttank 9 through a pay-off reel 1, a feeder leveller 2, a pinch roll andshearing device 3, a welder 4, the first bridle 5, an entry side looper6, the second bridle 7 and a tension unit 8. In the pre-treatment tank 9the strip is subjected to degreasing with alkali, washing in water,pickling, washing in water and water scrubbing. Degreasing and picklingmay be conducted by either the dipping method or the electrolytic methoddependent on the conditions of the surface of the steel strip. After thepre-treatment hot air is blown onto the strip from a drier 10 to rapidlydry the surfaces thereof in order to prevent rusting and to facilitatepainting of a slurry of aluminum powder and water in the subsequentprocess.

The dried strip (A) is fed to a slurry coater 12 through the thirdbridle 11 to be coated with a slurry consisting of aluminum powder andwater. The reverse type slurry coated is provided with devices formixing aluminum powder and water and for circulating slurry to supplyprepared slurry to the coater. As the slurry may be left as prepared fora long time before coating, there is a possibility that aluminum powderwill react with water thereby making use of the slurry impossible. Toprevent such a reaction, such slurry stabilizing agents as phytinicacid, aluminum chloride, aluminum sulfate and malonic acid are added tothe slurry. When steel strip is fed at a slow speed, rust may generateon the strip during the process time between coating of the strip withslurry and drying. Such rust is effectively prevented by addition of acorrosion inhibitor such as sodium nitrite and sodium benzoate.

The strip (A), coated with slurry, is fed to a rapid heating dryingfurnace 13 from which the evaporating water is rapidly exhausted, inorder to complete drying in as short a time as possible; the degree ofdrying must be uniform at the upper and lower surfaces of the stripwithout looping of the strips in the furnace; and exfoliation of driedaluminum powder caused by a supporting member for said looping must beprevented. A high frequency induction heating furnace provided with adevice for exhausting vapour meets the foregoing requirements. Thestructure of the heating furnace 13 will be described later in thespecification.

The steel strip delivered from the drying furnace 13 is then fed to arolling mill 14 where aluminum powder is completed and adhered to thesurface of the steel strip. In rolling the strip, a considerable amountof the aluminum powder sticks to the rolls of the mill 14 not onlyspoiling the surface of the steel strip but also damaging the uniformityof the coated metal. In order to prevent such defects, hollow rolls thesurfaces of which are cooled from inside thereof are used. In addition,a roll cleaning apparatus is provided at the delivery side of the rollsto clean the surface of the rolls and remove aluminum powder stuckthereto.

The steel strip delivered from the rolling mill 14 is then fed to theheat treatment furnace 15, 16 wherein the strip is rapidly andcontinuously subjected to a heat treatment to impart to the aluminumlayer resistance to bending, drawing, etc. Time and temperature for heattreatment depend on the thickness of the strip and the aluminum layerthereon. It is necessary to provide for rapidly changing temperature forheat treatment corresponding to changes in the production schedule. Inone embodiment of the present invention a continuous high frequencyinduction heating furnace 15 and radiation tube type furnace 16 areemployed as heating furnaces, but the invention is not limited thereto.In the embodiment, when the strip is thin and is rapidly coated withslurry, only the high frequency induction heating furnace 15 (FIGS. 4and 5) is employed while when the strip is thick and coated with slurryat a slow speed and must be heated for a long time, the radiation tubetype furnace 16 as well as said high frequency induction furnace 15 isemployed, as shown in FIGS. 7-9, to attain maintenance of desiredtemperature. In this way, temperature for heat treatment is adjustedcontinuously corresponding to the change of conditions for heattreatment caused by change in strip thickness and speed of coating bymeans of the combination of these two furnaces. Consequently, it ispossible to conduct heat treatment in a simple manufacturing line andreduce the construction cost.

The steel strip delivered from the heat treatment furnaces l5, 16 is fedto a water quenching tank 17 which is provided with a water circulationdevice having a spray nozzle to maintain the temperature of water at theentry side of the tank 17 at a constant value and with a wringer roll atthe delivery side of the tank. The temperature of water for cooling thestrip is adjustable in accordance with the requirements of the dryingprocess. The strip is further fed to a post treatment tank 19, to impartto the strip a corrosion resistant property, via a drier 18. The drier18 is used dependent upon the kind of chemical treatments to which thestrip is subjected in said tank 19. The method of post treatment may beselected from among chromate treatment, chromate treatment with solubleorganic high molecular compound, etc., by immersing or spraying.

The strip delivered from the post treatment tank 19 passes through a hotair blowing type drier 20, the fourth bridle 21, a delivery side looper22, the fifth bridle 23 and a delivery side shear 24, and is fed to atension reel 25 where the strip is coiled as a final product.

As has been described so far, the method of the present inventionshortens drying and heat treatment processes, thus bringing about rapidand continuous manufacturing of aluminum-coated steel plates.

Rapid heat treatment will be described in further detail hereinafter.

FIG. 2 shows the relation between temperature of a steel plate andtreatment time. The upper line in FIG. 2 represents the upper limit ofheat treatment temperature for not forming an alloy layer of Fe and Alin the interface of the aluminum layer and the steel plate. If the hardand brittle alloy layer of Fe and A1 is formed, cracks are easily causedin the coated layer on a steel plate while it is being formed. Moreover,aluminumcoated steel plate, treated under such conditions as to form anFe Al alloy layer, does not have the lustre peculiar to aluminum andbecomes yellowish in color. Consequently, heat treatment at atemperature above the aforementioned upper limit is undesirable.

The lower line shown in FIG. 2 represents the lower limit of heattreatment temperature below which adherence between the aluminum layerand the steel plate is not sufficient and exfoliation of the aluminumlayer or minute cracks therein are caused during working of thealuminum-coated steel plate. Thus, the intermediate part between theupper and lower lines shown in FIG. 2 is the optimum range for the heattreatment. Aluminum-coated steel plates having excellent properties, inwhich no cracks are caused in the aluminum layer, are obtained by heattreatment under the conditions within the above range. By furtherexamining the said range to obtain better and more economical conditionsfor a continuous process of manufacturing aluminum-coated steel platecomprising painting, roll and heat treatment, the range (B) indicated byoblique lines in FIG. 2 is obtained. The present invention ischaracterized by rapid heat treatment under the conditions within therange (B).

Heat treatment under such conditions is impossible by the conventionalheating method with gases.

In order to attain such a heat treatment for the present invention,suitable heating methods are: (a) heating aluminum-coated steel plateswith a metal having both good heat conductivity and not reacting withaluminum; (b) heating by means of induction heating as described inreference to FIG. 1; or (c) rapidly heating the steel plate up to apredetermined temperature by induction heating and maintaining the sameat the temperature in a furnace containing gas therein. Rapid heatconduction is effected by a metal bath, such as a lead bath or aninduction heating furnace, and steel plates are heated up to apredetermined temperature in a short time. FIG. 3 shows the manner inwhich the temperature rises in some heating furnaces. In FIG. 3, (a)shows a case of heating with a lead bath, (b) heating in an inductionheating furnace, (c) in a combination of an induction heating furnaceand a siliconit furnace and (d) in a siliconit furnace.

Other embodiments of the present invention will be describedhereinafter.

EMBODIMENT 2 A slurry consisting of atomized aluminum powder comprisingparticles having an average particle size of 15 1.1. and an aqueoussolution of NaNO having a concentration of IO moIe/l, mixed in the ratioof 6:4, was coated on a steel plate by means of a reverse roll coater.The coating rate was 80 g/m Then the steel plate was dried and rolled ina rolling mill so as to reduce the steel plate by 5 percent. Thealuminumcoated steel plate thus obtained was subjected to oil quenchingafter heat treatment for 3 seconds in a lead bath which was maintainedat 600C. The steel plate thus obtained had the metallic luster peculiarto aluminum and no cracks were caused in the coated layer in bendingtests to an angle of 180. In a salt spraying test (Japanese IndustrialStandard Z -2371), red rusts did not appear at all, even after beingsubjected to a test for 200 hours. Thus, it is shown that the steelplate has an excellent corrosion resistant property.

EMBODIMENT 3 A rolled aluminum-coated steel plate obtained in the sameway as Embodiment 2 was subjected to heat treatment for 5 seconds in alead bath maintained at 550C, and then to water quenching.Aluminum-coated steel plate having the same properties as Embodiment 2was obtained.

EMBODIMENT 4 A slurry comprising the same compositions as that ofEmbodiment 2 was continuously coated on a coldrolled coil having athickness of 0.8 mm and a width of 100 mm by a reverse system rollcoated at a rate of dried weight of g/m and the coated coil was driedand rolled to 5 percent reduction rate. The coil was then heated for 2seconds at a heating rate of 225C/sec with an input power of 7 KW in aninduction heating furnace having frequency of 10 KC, maintained at 550Cfor 3 seconds with 0.5 KW and subjected to water quenching.

Aluminum-coated steel plate thus obtained had the same excellentproperties as Embodiment 2, having beautiful luster, excellentworkability and resistance to corrosion.

EMBODIMENT 5 The rolled aluminum-coated steel plate obtained in the sameway as Embodiment 4 was heated for 2 seconds in such a way that thetemperature at the surface of the steel plate became 500C with an inputpower of 7 KW in an induction heating furnace having a frequency of 10KC and then heated for 3 seconds at 500C in a furnace in the atmosphere.The steel plate thus obtained has luster on the surface thereof and nodefects were seen in the coated layer in a deep drawing test. Thoughwhite rust was caused in a humidity cabinet test at 50C and with percentof moisture, red rust did not generate after a test for 3000 hours.

As has been described so far, aluminum powder coated on a steel plate iscompacted to provide almost theoretical density, and subjected to heattreatment at a temperature within the range from 450 to 700C for l5seconds. As the result of the foregoing, time for heat treatment isremarkably shortened. Further, it is possible to shorten furnace lengthand to conduct rapid and continuous operation. Moreover, operation iseasily performed at low cost because heat treatment is conducted in theatmosphere and heating in a nonoxidizing or reducing atmosphere isunnecessary. Heat treatment under these particular conditions preventsforming a hard and brittle alloy layer, damaging the luster peculiar toaluminum or causing insufficient adherence.

The present invention is characterized by the abovementioned rapid heattreatment. Embodiments of heating furnaces will be describedhereinafter, reference being made to FIGS. 4-9 of the accompanyingdrawing.

In the drawing, 40 denotes a steel plate coated on both surfaces with aslurry consisting of aluminum powder and water, which is fed to aheating furnace 41 as shown in FIGS. 4 and 7. The ,heating furnace isequipped with a hot air blowing device 42 at the entry side thereof,where the hot air heated up to 80l00C is applied to the steel plateentering the heating furnace 41 to accelerate drying. 43a, 43b and 430denote coils for high frequency induction heating. Inductance at therespective coils 43a, 43b and 430 is continuously adjusted to facilitatecontinuously varying the curve of temperatures at the respective coilsindependently of each other by means of respective control systems whichare respectively independent of each other. Thus, input power to therespective coils for heating is made adjustable at loading. As shown inFIG. 5, the coils 43 comprise an outer protective insulating wall 44 formechanical portection of the coil and prevention of dust and danger, andan inner protective insulating wall 45 which is exchangeable formechanical protection of the coils 43 when the strip 40 is damaged andto prevent slurry which is not dried yet from dropping directly on theinside of the coils 43 and adhering thereto. Connecting devices forexhaust 46a, 46b and 46c are provided at the delivery side of theheating furnace 41 and between the respective coils 43a, 43b and 43c.These exhaust devices 46 which may be connected to exhaust blowersaccelerate drying by exhausting vapours and gases which are generatedupon heating from the top and bottom ends of the devices 46. Windows47a,-47b and 470 (FIGS. 4 and 8) are provided in the connecting devices46a, 46b and 460 to enable one to observe the state of drying within theheating coils 43 so that the load on the heating coils can be adjustedif necessary. FIGS. 7 and 9 show radiation tubes 48 in the furnace toadditionally provide radiation heating as hereinbefore described.

As is described hereinabove, the high frequency induction heatingfurnace of the present invention comprises a plurality of high frequencyinduction heating coils which facilitate independent control of thetemperature curve by means of the control systems which are independentof each other, the hot air blowing device at the entry side of theheating furnace, and the connecting devices for exhaust betweenrespective coils and at the delivery side of the furnace. Further,radiation heating is supplied. Thus, a great deal of vapour and gasgenerated when slurry containing alminum powder coated on the steelstrip is dried is exhausted through the exhaust device to promote dryingand effective heating. At the time of drying and of heat treatment afterrolling, heating speed and temperature are adjusted to provide theoptimum conditions for each coil depending upon the thickness of thestrip which is being treated, the number of heating coils which can becontrolled independently of each other, the kind of coated metals andthe thickness of coated layer.

An embodiment of rolling rolls which are employed in the rolling processwill be described hereinafter, reference being made to FIG. 6.

In FIG. 6, 51 denotes a lower work roll and 52 denotes an upper workroll. A steel strip coated with aluminum powder and subjected to heattreatment is rolled between said upper and lower rolls 51, 52. 53denotes ball bearings to support said rolls 51, 52.

The rolls 51, 52 are provided with a hollow portion 54 extending in theaxial direction and one end of which is closed, in which a water supplytube 55 supported by a receiving member 56 at the closed end of thehollow portion 54 is provided in the centerthereof. A plurality of watersupply apertures 57 are provided in the water supply tube 55. Coolingwater is supplied to the inside of the hollow portion 54 through theapertures 57 of the water supply tube 55 to cool the rolls 51, 52.

58 denotes a rotary joint connected to the open end of the hollowportion 54 in the rolls 51, 52, comprising double rotary sleeves 60, 61installed in the fixed casing 59, ball bearings 62, 63 to support saidtwo rotary sleeves 60, 61, and flexible hoses 64, 65 to supply saidsleeves with cooling water.

The end of the rolls 51, 52 is fixed to the outer rotary sleeve in aliquid-tight manner, and the gap between the outer rotary sleeve 60 andthe inner rotary sleeve 61 is connected to the hollow portion 54 and theflexible hose for drainage of cooling water supplied in the hollowportion 54. The water supply tube 55 is connected to the inner rotarysleeve 61 at one end thereof and the other end of the inner rotarysleeve 61 is opened to a cover 66 which is fixed to the fixed casing 59and in which the flexible hose 64 for supplying water is connected.

Cooling water supplied through the flexible hose 64 is fed to the watersupply tube 55 through the inner rotary sleeve 61 and to the hollowportion 54 from the water supply apertures 57. Water fed to the hollowportion 54 and cooling the rolls 51, 52 is drained from the flexiblehose 65 through the gap 67 in the outer rotary sleeve 60.

As above-described, the rolls are cooled from the inside thereof.Therefore, coated aluminum powder is not washed away by cooling water orlubricant oil as often happened in the prior art, nor does it adhere tothe rolls and damage the uniform surface of the rolled steel strip.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A heating furnace for use in the manufacture of steel plates coatedwith aluminum powder, comprising:

housing means for receiving a moving steel plate coated with aluminumpowder;

a high frequency induction heating means in said housing means forheating said steel plate, said heated steel plate conducting heatgradually to the outer portion of said coating;

a radiation type heating means in said housing means for heating saidcoating, the heated coating conducting heat gradually toward the insidethereof;

a hot air blowing device at the entry side of said housing means forsupplying fresh air to said furnace; and

exhaust means coupled to said housing means for removing wetted hot airfrom said furnace.

2. A heating furnace according to claim 1 wherein said exhaust means islocated at least at the delivery side of said housing means.

3. A heating furnace according to claim 2 wherein said exhaust meansincludes a plurality of outlets for exhausting gas from said furnace atdifferent portions thereof.

4. A heating furnace according to claim 1 wherein said high frequencyinduction heating means comprises a plurality of high frequencyinduction heaters spaced from each other and distributed along saidhousing means in the'direction of movement of said steel plate.

5. A heating furnace according to claim 4 wherein said radiation typeheating means comprises a plurality of radiation heaters spaced fromeach other and distributed along said housing means in the direction ofmovement of said steel plate.

6. A heating furnace according to claim 1 wherein said housing meansincludes openings therein for observance of the conditions inside saidfurnace.

said plurality of high frequency induction heating means for controllingthe heating temperature thereof so as to control the temperaturegradient in said furnace;

a hot air blowing device at the entry side of said housing means forsupplying fresh air to said furnace; and

exhaust means coupled to said housing means for removing wetted hot airfrom said furnace.

1. A HEATING FURNACE FOR USE IN THE MANUFACTURER OF STEEL PLATES COATEDWITH ALUMINUM POWDER, COMPRISING: HOUSING MEANS FOR RECEIVING A MOVINGSTEEL PLATE COATED WITH ALUNIMUM POWDER, A HIGH FREQUENCY INDUCTIONHEATING MEANS IN SAID HOUSING MEANS FOR HEATING SAID STEEL PLATE, SAIDHEATED STEEL PLATE CONDUCTING HEAT GRADUALLY TO THE OUTER PORTION OFSAID COATING, A RADIATION TYPE HEATING MEANS IN SAID HOUSING MEANS FORHEATING SAID COATING, THEHEATED COATING CONDUCTING HEAT GRADUALLY TOWARDTHE INSIDE THEREOF, A HOT AIR BLOWING DEVICE AT THE ENTRY SIDE OF SAIDHOUSING MEANS FOR SUPPLYING FRESH AIR TO SAID FURNACE, AND EXHAUST MEANSCOUPLED TO SAID HOUSING MEANS FOR REMOVING WETTED HOT AIR FROM SAIDFURNACE.
 2. A heating furnace according to claim 1 wherein said exhaustmeans is located at least at the delivery side of said housing means. 3.A heating furnace according to claim 2 wherein said exhaust meansincludes a plurality of outlets for exhausting gaS from said furnace atdifferent portions thereof.
 4. A heating furnace according to claim 1wherein said high frequency induction heating means comprises aplurality of high frequency induction heaters spaced from each other anddistributed along said housing means in the direction of movement ofsaid steel plate.
 5. A heating furnace according to claim 4 wherein saidradiation type heating means comprises a plurality of radiation heatersspaced from each other and distributed along said housing means in thedirection of movement of said steel plate.
 6. A heating furnaceaccording to claim 1 wherein said housing means includes openingstherein for observance of the conditions inside said furnace.
 7. Aheating furnace according to claim 6 wherein said housing means includesa plurality of said openings spaced from each other along the directionof movement of said steel plate.
 8. A heating furnace for use in themanufacture of steel plates coated with aluminum powder, comprising:housing means for receiving a moving steel plate coated with aluminumpowder; a plurality of high frequency induction heating means in saidhousing means; an independent control system coupled to each of saidplurality of high frequency induction heating means for controlling theheating temperature thereof so as to control the temperature gradient insaid furnace; a hot air blowing device at the entry side of said housingmeans for supplying fresh air to said furnace; and exhaust means coupledto said housing means for removing wetted hot air from said furnace.